Studies have been initiated to isolate and examine the genes encoding type 1 and type 2 fimbriae from Actinomyces viscosus in order to investigate the structural organization and biosynthesis of these fimbriae. Cosmid cloning was used to establish a gene library of A. viscosus T14V in Escherichia coli. Restriction analysis showed that on a statistical basis (P greater than 0.99) this library of 550 clones represented the entire A. Viscosus genome. Colony immunoassays showed that 25 clones reacted with antiserum directed against A. viscosus cells. The proteins responsible for these reactions were identified in 3 clones. Clones AV1209 and 2009 expressed non-fimbrial antigens of 40 and 58 kilodalton, respectively. Clone AV1402 expressed a 59 kilodalton protein that reacted with monospecific antiserum against type 2 fimbriae; this protein was identical in size (as judged by electrophoretic mobility) to a component of type 2 fimbriae. Thus, a gene (fim A) encoding a type 2 fimbrial subunit has been isolated and is expressed in E. coli.